1. Field of the Invention
The present invention relates to a global positioning system (GPS) receiver which receives a signal from a GPS satellite in measurement of a present position to measure the precise position, while capable of securely obtaining data on the present position, and to a GPS reception method using same.
2. Description of the Related Art
In recent years, navigation apparatus widely used in vehicles have employed one or both of a autonomous navigation method and a GPS navigation method as a measurement method in measurement of a position of a vehicle, the combination of these navigation methods being referred to as a hybrid navigation method. The autonomous navigation method involves calculating a travel distance from signals indicative of a vehicle speed or based on an acceleration sensor, and determining a rotation angle of the vehicle based on an azimuth detection sensor such as an angular speed sensor or the like, thereby constantly adding the travel direction taken and the traveled distance to those previously determined. The GPS navigation method involves receiving electric waves from a GPS satellite to detect the present position.
To display the present position of the vehicle and the travel direction thereof thus determined on a map screen in the navigation apparatus, map data about a map covering the present position and its surroundings is read from a map data storage medium, such as a DVD-ROM, installed in the navigation apparatus, and an electronic map associated with the map data is displayed on a monitor screen with a mark indicative of the vehicle position being superimposed and displayed on the map. The navigation apparatus has a map matching function of matching a measured position of the vehicle onto an appropriate map route by comparison with data about roads on the map so as to display the vehicle position on the map. When there is a plurality of roads taken as a candidate after determination of the position of a moving member, this map matching function is performed to select a road with the highest existing probability from among the roads as the appropriate road, for example, by giving a high priority to the route along which the vehicle is being guided.
In such a navigation apparatus, it is very important to accurately measure the present position of the vehicle. In particular, in cases where the data mainly used is reception data from the GPS satellites, the GPS data itself to be received must be precise, and the GPS satellite constellation needed for the measurement must be appropriate. In receiver techniques, the so-called “all-in-view” mode in which all visible satellites are used for positioning calculation has recently become mainstream. It is further very important to select appropriate satellites from among all the visible satellites.
Note that the above-mentioned GPS receiver for receiving the signals of the GPS satellites to measure the present position is used not only for the above navigation apparatus mounted on the vehicle or the like, but for other devices, such as a personal computer, a portable information terminal, a portable telephone, or the like. Thus, the GPS receiver is widely used in various fields for precise measurement of the present position and displaying it on the map screen.
For measurement of the present position using the GPS satellites as described above, the reception signals including the GPS data to be received must be precise, while the signals actually received from the GPS satellites may be degraded by attenuation due to an inappropriate distance from the GPS satellite or due to obstacles, such as clouds high in the sky, or by occurrence of multipath noise due to the influence of surrounding buildings or the like. Since the large electric field strength of electric waves from the GPS satellite enables precise detection of the position, an S/N value associated with the electric field strength of the electric waves received from the GPS receiver is measured. Then, an S/N threshold is set such that the GPS data with the S/N value less than a predetermined value is not used for the positional detection. On the other hand, when the threshold is set to the higher value so as to use only the highly reliable signals of the GPS satellites, the number of the GPS satellites which can be used for the positional detection becomes decreased.
An influence on the measurement precision or accuracy of the GPS satellite constellation needed for measurement is represented by a dilution of precision (DOP) value (hereinafter referred to as a DOP value). The DOP value indicates a numerical value representing the arrangement or constellation of a plurality of GPS satellites used for measurement of the present position. For example, when three or more of the GPS satellites are used to detect the present position, the DOP value is represented by the value of an area enclosed by the plurality of GPS satellites. The smaller the DOP value, the more accurate the position detection can be.
In selection of a combination of the satellites with the smallest DOP value from among the visible satellites, if the smallest DOP value of the combination is larger than a predetermined value, accurate positional data is not available. Thus, the data obtained from the satellites whose DOP value is larger than a predetermined threshold is not used. For this reason, setting the threshold to the smaller value enables usage of the more accurate data, but the combination of the satellites which satisfy the threshold may not often be obtained, thus resulting in failure in detection of the present position by the GPS satellites.
It should be noted that there are proposed various types of techniques for performing appropriate position detection considering the above-mentioned DOP value, and that one of these techniques is disclosed in a JP-A-6-18644. This technique involves calculating a DOP value of each of the possible combinations of a predetermined number of visible satellites at predetermined calculation time intervals, and selecting the combination of the satellites with the smallest DOP value. In performing positioning calculation based on the reception data of the satellites constituting the combination selected, the limit of the DOP value is pre-set. Even if a combination of the satellites other than the combination previously selected has the minimum DOP value at the DOP value calculation timing, as long as the DOP value of the combination previously selected does not exceed the limit pre-set, the combination selected is maintained for the positioning calculation thereby to prevent changes in data due to a change from the selected combination of the satellites for the positioning calculation to another for a short period of time, thus resulting in a smooth track of the present position obtained from the positioning calculation.
In the above-mentioned GPS receiver used, for example, in the navigation system, the personal computer, the portable information terminal, the portable telephone, or the like, it is important to accurately determine the present position. In addition, the GPS receiving system is required to be able to measure the position with high accuracy using the high-quality signals and by use of the satellites with high probability. However, the noise effect, the obstacles in a propagation path, or the multipath effect may degrade the reception signals, which include error components, leading to an increased error in the position measured.
On the other hand, in some cases, the reception signals may not be used for the positioning calculation due to low level thereof, and the DOP value of the satellite constellation may become so large at the timing of positioning calculation, whereby the reliability of result of the positioning calculation may become low, and the positioning solution cannot be utilized.
These two phenomena have the characteristics opposite to each other. To aim for the high-accuracy positioning, tightening adoption criteria of signals to enhance the quality thereof, or setting strictly the DOP value that serves as an adoption criterion of a positioning solution leads to a low probability of positioning or measurement. In contrast, lowering criteria of signal quality for increasing the probability of positioning, or setting widely the criterion of the DOP value can improve the positioning probability, but results in degradation in the measurement accuracy. These problems are mutually contradictory.
This situation is illustrated in FIG. 3. That is, as shown in FIG. 3, in a GPS system mounted on a vehicle C, GPS satellites currently receivable are designated by reference numerals S1 to S8, and distances from the respective GPS satellites to the vehicle are designated by reference numerals r1 to r8. The GPS satellite S1 is affected by a nearby construction B, and the GPS satellites S5 to S8 are affected by a forest F, resulting in degradation in reception signals therefrom. Since only signals from three GPS satellites S2 to S4 satisfy a threshold value previously set, only these three GPS satellite signals are used to measure the position. Alternatively, if the three GPS signals are not sufficient to allow the appropriate positioning, then the position measurement by the GPS satellites may not be taken until four GPS satellite signals can respectively satisfy the threshold after the vehicle further moves. As mentioned above, heightening the threshold value with respect to the reception signal from the GPS satellite for the accurate position measurement may result in decreased number of the GPS satellites that meet the threshold. This may often fail to measure the position using the GPS satellites.
The same goes for the following situation. That is, in addition to the level of reception signals from the respective GPS satellites, when only the combination of the GPS satellites are selected whose DOP value based on the positional relationship among the GPS satellites exceeds a pre-set threshold value, heightening the DOP threshold value may also lead to decrease in the number of the GPS satellites that satisfy the threshold.
Thus, in the conventional GPS receivers, the threshold value of the level of reception signal from each of the GPS satellites is set and fixed, and the DOP value based on an interrelation between the satellites is also set and fixed in selection of the appropriate GPS satellites from all visible satellites for reception of the signals from the GPS satellites and for performing positioning. Tightening up these thresholds to improve the measurement accuracy often fails to perform the positioning using the GPS satellites. Especially, the recent GPS receiver is required to measure the position with high sensitivity and accuracy, taking into consideration an application to the portable telephone.